The present invention relates to method and apparatus for controlling the heat and mass inventory in a reactant plenum.
One of the major problems encountered in the development of fuel cells which employ oxygen containing gases as the oxidant and hydrogen containing gases as the fuel, is the removal of heat and water produced as a result of oxidizing the fuels. In order to maintain a steady state operation the reaction product must be removed at the same rate at which it is formed and an exchange of heat with the surroundings must take place. Any imbalance between the production and recovery rates of the reaction products can detrimentally affect the performance of the cell. For example, if product water is not adequately removed from a H.sub.2 --O.sub.2 fuel cell the concentration of the electrolyte can be diluted and electrode flooding may even occur. Either of these events would dramatically alter the cell performance. In other types of cells, such as those using an ion exchange membrane as an electrolyte, a decrease in water vapor concentration in the cell can result in a drying out of the electrolyte and cell failure. Similarly, variations in temperature significantly reduce the performance of the cell through a combination of factors including changes in the vapor pressure of the electrolyte and changes in the rate of reaction.
Various means have been proposed for removing water and controlling the temperature within fuel cells. Thus, the art is replete with different arrangements of pumps and condensors for removing water and circulating coolant through the fuel cell. Systems of these types, of course, require significant power for their operation and consequently, reduce the otherwise high efficiency of fuel cells.
In some arrangements, heat exchange plates have been incorporated in fuel cells for transporting heat to the surface of fins extending external the fuel cell assembly. Not only are metal heat exchange plates heavy and cumbersome but unfortunately temperature gradients exist over the surface of the plate making precise and uniform temperature control virtually impossible.
Interestingly, heat control and mass inventory in fuel cells are generally handled by separate and distinct systems. In U.S. Pat. No. 3,370,984, for example, water is removed from the fuel cell by transport through a membrane and into a vapor cavity where it is expelled. Some of the heat produced in the cell is used to evaporate part of the water produced in the cell but this only accounts for about 20% of the total heat that must be removed. Consequently, cooling plates and fins or some other device is required for temperature control.
In addition to reducing the efficiency of fuel cells many of the prior art systems for controlling temperature and mass inventory within the cell tend to be large as well as expensive. In today's age of miniaturization sizing can be an acute problem.